1. Technical Field of the Invention
The present invention relates to spread spectrum communications systems and, in particular, to the despreading of direct sequence spread spectrum communications signals.
2. Description of Related Art
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is outstripping system capacity. If this trend continues, the effects of rapid growth will soon reach even the smallest markets. The predominant problem with respect to continued growth is that the customer base is expanding while the amount of electromagnetic spectrum allocated to cellular service providers remains fixed. Innovative solutions are required to meet these increasing capacity needs as well as to maintain high quality service and avoid rising prices.
Currently, channel access is primarily achieved using Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) methods. In frequency division multiple access systems, a communication channel is a single radio frequency band into which the transmission power of a signal is concentrated. In time division multiple access systems, a channel comprises a time slot in a periodic train of time intervals over the same radio frequency.
Spread spectrum comprises a communications technique that is finding commercial application in wireless communications. Spread spectrum systems have been around since the days of World War II. Early applications were predominantly military oriented (relating to smart jamming and radar). However, there is an increasing interest today in using spread spectrum systems in commercial applications, including digital cellular radio, land mobile radio, and indoor and outdoor personal communication networks.
In a spread spectrum transmitter, a digital bit stream at a basic data rate is spread to a transmit data rate (or chip rate). This spreading operation involves applying a user unique digital code (the spreading or signature sequence) to the bit stream that increases its data rate while adding redundancy. This application typically multiplies (or logically XOR's) the digital bit stream by the digital code. The resulting transmitted data sequences (chips) are then modulated using a form of quadrature phase shift keying (QPSK) to generate an output signal. This output signal is added to other similarly processed output signals for multi-channel transmission over a communications medium. The output signals of multiple users (channels) advantageously share one transmission communications frequency, with the multiple signals appearing to be located on top of each other in both the frequency domain and the time domain. Because the applied digital codes are user unique, however, each output signal transmitted over the shared communications frequency is similarly unique, and through the application of proper processing techniques at the receiver may be distinguished from each other. In the spread spectrum receiver, the received signals are demodulated and the appropriate digital code for the user of interest is applied (i.e., multiplied) to despread, or remove the coding from the desired transmitted signal and return to the basic data rate. Where this digital code is applied to other transmitted and received signals, however, there is no despreading as the signals maintain their chip rate. The despreading operation thus effectively comprises a correlation process comparing the received signal with the appropriate digital code.
In many spread spectrum communication systems, the transmitted data sequences include two components: an in-phase (I) component and a quadrature phase (Q) component. These components are typically viewed as the real and imaginary parts of a complex signal. In the spread spectrum transmitter, a complex spreading sequence is applied, and the two components are modulated (in accordance with the quadrature phase shift keying processing) and combined to form the generated output signal for transmission. Because the received signal now similarly includes both an in-phase component and a quadrature phase component, the despreading operation performed by the spread spectrum receiver must correlate the received complex signal to the appropriate digital code (signature sequence). This is typically accomplished using two scalar correlators, one fed with in-phase samples and one fed with quadrature phase samples. If a complex spreading sequence is used, however, four scalar correlators are needed further increasing the complexity of the correlation process.
The signals transmitted between two places in mobile communication systems may suffer from echo distortion or time dispersion. Multipath dispersion occurs when a signal proceeds to the receiver along not one, but many paths, so that the receiver receives many echoes having different and randomly varying delays and amplitudes. This is typically caused by, for example, signal reflections from large buildings or nearby mountain ranges. When multipath time dispersion is present in a spread spectrum communications system, the received signal comprises a composite of multiple versions (or images) of the transmitted signal that have propagated along different paths (referred to as "rays"). These versions of the transmitted signal typically have relative time delays with respect to each other of less than one symbol period. In certain situations, like macro-diversity and soft handoff, the delays may be greater.
The presence of multipath time dispersion with respect to a complex transmitted spread spectrum signal significantly complicates the receiving and correlating processes. A RAKE receiver, so named because it rakes the multipath contributions together using a weighted sum, is then used to receive the multiple rays of the signal. One correlation device (comprising an in-phase correlation section and a quadrature phase correlation section) is provided for each image of the transmitted signal. Each correlation device is aligned with its corresponding signal image (ray) through the use of a delay line. The received time diverse signals at each correlation device are then weighted in proportion to their received signal amplitudes, and the resulting signals summed (in-phase and quadrature phase) for output and further processing.
It is further noted that in some systems the overall spreading sequence may in fact comprise a combination of multiple sequences. For example, in the digital cellular standard TIA IS-95 for code division multiple access (CDMA) spread spectrum communications, downlink information is spread by using a real signature sequence. The information is further scrambled with in-phase and quadrature phase scrambling sequences. Thus, the overall spreading sequence is complex, resulting from a composite of a complex scrambling sequence on top of a real spreading sequence. At the receiver, complex correlations are needed to process the signal giving rise to a high degree of complexity when using conventional techniques. The complexity is further increased when multiple channels (for example, traffic and pilot) need to be simultaneously despread.
There is a need then for reduced complexity correlation architectures for processing complex spread spectrum communications signals subject to multipath time dispersion, composite spreading and multi-channel reception.